Refracting,light permeable oxide layer and method for its manufacture

ABSTRACT

A HIGHLY REFRACTING, LIGHT PERMEABLE OXIDE LAYER ON A SUPPORT, SUCH AS A GLASS PLATE, COMPRISES OXYGEN, ZIRCONIUM AND TANTALUMN, THE RATIO OF THE NUMBER OF TANTALUM ATOMS TO ZIRCONIUM ATOMS BEING IN THE RANGE OF ABOUT 1:1 TO 1:2.

Jall- 1974 B. WILLE 3,783,010

HEFRACTING. lJIGHT FERMBABLJE OXIDE LAYER AND METHOD FOR ITS MANUFACTURE Filed April 7, 1971 INVENTOR. 54/ 7/57 M4 15 3,783,010 REFRACTING, LIGHT PERMEABLE OXIDE LAYER AND METHOD FOR ITS MANUFACTURE Baptist Wille, Balzers, Liechtenstein, assignor to Balzers Patentund Beteiligungs Aktiengeselischaft, Balzers, Liechtenstein Filed Apr. 7, 1971, Ser. No. 132,041 Claims priority, application Switzerland, Apr. 13, 1970, 5,491/70 lint. Cl. C23c 11/00, 13/00 US. Cl. 117-106 R 2 Claims ABSTRACT OF THE DISCLOSURE A highly retracting, light permeable oxide layer on a support, such as a glass plate, comprises oxygen, zirconium and tantalum, the ratio of the number of tantalum atoms to zirconium atoms being in the range of about 1:1 to 1:2.

FIELD OF INVENTION The invention relates to the production of optical layers by vacuum deposition.

BACKGROUND INFORMATION AND PRIOR ART It is well known that thin oxide layers are used to a very large extent and for various purposes as protective layers and as layers for optical purposes. Oxidic protecting layers serve, for example, the purpose to protect sensitive surfaces of, for example, lenses, surface mirrors and the like objects against corrosion and mechanical injury or damage. In the optical industry, oxidic layers are commonly used as reflection reducing coatings, interference filters, beam or ray dividers, heat filters, cold light mirrors and coatings for spectacle lenses and the like. As is well known in the art, the mechanical and optical characteristics of such oxidic layers are dependent not only on the nature of the oxide which has been deposited on the respective surface, but also to a very pronounced extent on the manner in which the layer has been deposited. Although considerable research has been done in this field, the choice for suitable, highly refractive layers for optical purposes is very small, particularly if the layer is to meet stringent requirements in respect to homogeneity and nonabsorption.

It is known to produce oxide layers by direct evaporation of oxidic starting materials in a vacuum and subsequent condensation of the vapors on the support to be coated. This commonly used procedure has considerable disadvantages if the deposited layer is to be used for optical purposes because most oxides upon evaporation and subsequent condensation under vacuum conditions yield absorbing layers. This also holds true if the starting materials are absorption-free oxides. At the present state of'the art, it is known that the reason for this phenomenon is that most oxides are reduced by the evaporation in a vacuum, and the unsaturated oxides exhibit optical absorption. Only a few exceptions are known in this respect: Thus for example, SiO can be vaporized in a vacuum by means of an electron beam to produce absorption-free layers.

atent With a view to circumnavigating these difliculties in the production of absorption-free oxide layers, a number of other processes has been developed. It has thus been suggested to produce absorption-free metal oxide layers by vacuum deposition of the respective metals and subsequent oxidation of the metals or by cathode atomization of the metals in an oxygen atmosphere. It has also been proposed to deposit the starting substances in an oxidizing atmosphere.

According to another known procedure for the production of absorption-poor oxide layers by vaporizing an oxidic starting substance or a starting substance which is capable of being oxidized, the starting substance or substances is or are admixed with one or several rare earth elements or compounds.

For many optical application of layers which have been deposited by vapor deposition, it is important that the layers are homogeneous, to wit, that their optical characteristics are uniform throughout all portions of the layer. This is so because layer systems, for example, refiection reducing coatings which consist of two layers are almost always calculated under the assumption that the system consists of homogeneous layers because nonhomogeneous layers cannot be produced in a reproducible manner. For the manufacture of such layer systems it is then a precondition that the homogeneous layers which form the basis for the calculation are in fact present.

It is an extremely disturbing factor in interference layer systems that many layers have the characteristic that their index of refraction is not constant in the direction prependicular to the plane of the layer. In the following, the layers in which the index of refraction decreases in the course of the layer formation, to wit, with increasing layer thickness, are referred to as negative-inhomogeneous while layers in which the index of refraction increases With increasing layer thickness, are referred to as positive-inhomogeneous. Experience has indicated that there is always a tendency towards inhomogeneity of the index of refraction, particularly with the presently known highly refracting layers and to the extent that such layers can be considered for optical-technical use due to their absorption and mechanical characteristics. The particular extent or degree of the inhomogeneity is subject to strong fluctuations, dependent on the manufacturing conditions.

Thus, for example, zirconium oxide layers which, due to their high index of refraction and their nonabsorptiveness in the visible spectral region and their mechanical characteristics have been proposed for the construction of optical layer systems, unfortunately exhibit a very pronounced negative inhomogeneity. This strongly reduces, for example, the effectiveness of a reflection-reducing coating composed of such a layer. Various attempts have been made to eliminate this negative inhomogeneity by changing the deposition procedure, but all these attempts have failed.

Some deposited layers exhibit a positive inhomogeneity. There are uses in the optical technology in which such positive inhomogeneity can be accepted, for example, in the highly refracting first layer of a reflectionreducing coating consisting of two layers.

SUMMARY OF THE INVENTION It is a primary object of the invention to overcome the disadvantages of the prior art oxidic layers and to produce a light permeable layer by vacuum deposition on a support which does not exhibit the undesired negative inhomogeneity characteristic, which has a high index of refraction and which is exceedingly resistant to mechanical and chemical influences.

Another object of the invention is to provide a method for producing a layer of this kind which is exceedingly simple to carry out with a minimum of expenditure.

It is also an object of the invention to provide a procedure for forming a superior optical layer on a support, wherein the starting materials can be readily vacuum-deposited on the support.

Generally, it is an object of the invention to improve on the art of vapor deposition of layers on supports.

Briefly, and in accordance with the invention, the above objects are superiorly achieved by providing a layer on a support, which layer essentially consists of oxygen, zirconium and tantalum, the ratio of the number of tantalum atoms to zirconium atoms being in the range of 1:1 to 1:2.

According to the inventive procedure for producing a highly retracting, light permeable oxide layer on a support, a mixture containing the elements zirconium and tantalum in metallic or oxidic form, at least one of the elements being present in oxidic form, is heated under vacuum conditions to vaporize the mixture, whereupon the vapor is condensed on the support to be coated.

The invention will now be described by a number of examples, it being understood, however, that these examples are given by way of illustration and not by way of limitation, and that many changes may be effected without aifecting in any way the scope and spirit of the invention as recited in the appended claims.

Example 1 This test was carried out in a heatable vaporizing crucible of a vacuum deposition plant. For the purpose of producing an optical layer, a mixture consisting of 30% of metallic tantalum, calculated on the weight of the total mixture, and 70% of zirconium oxide (ZrO were inserted into the crucible. The crucible was heated with a wattage of about 2.5 kw. This resulted in a temperature of the mixture in the crucible of about 1800 C. A support in the form of a glass plate was arranged in the plant opposite the crucible, and the heating resulted in vaporiza tion of the material in the crucible and the deposition of a layer of a thickness of 250 nm. with a growing speed of 35 nm. per minute. The temperature of the glass plate was about 300 C. The vapor deposition was carried out in known manner in an oxygen atmosphere of mm. Hg. A layer was obtained which adhered satisfactorily to the glass plate, which was hard and completely absorptionfree in the visible spectral region. The layer had a constant index of refraction of n=2.05 throughout its entire thickness.

The constant index of refraction can be easily confirmed by proceeding in the following manner:

A layer to be investigated is vapor-deposited on a glass plate with a low index of refraction until, upon illumination with light of a predetermined wave length, a minimum of the reflection occurs. If the deposited layer is homogeneous, then the reflection capability of the surface of the glass plate on which the layer has been deposited, should not differ from the respective wave length from the reflection capability of the uncoated glass surface. A layer is then present which conforms to the equation wherein A indicates the respective wave length, n is the index of refraction of the layer material and d is the thickness of the layer. By contrast, a higher reflection capability relative to the uncoated glass surface is obtained if the layer is positively inhomogeneous while a lower reflection capability as compared to that of the uncoated glass surface is obtained if the layer is negatively inhomogeneous.

Experiments have indicated that the invention is particularly advantageously carried out by using starting mixtures in which the element tantalum is present in metallic form while the element zirconium is employed in oxide form. The proportion of metallic tantalum should then not be below 20% of the weight of the total mixture. The same applies for the zirconium oxide component. Within these limits the layer produced from such mixtures is surprisingly insignificantly dependent on the mix- TABLE 1 Ratio oi number of atoms Ta/Zr in the- Weight ratio Layer 'IazZrOz in (X-raythe starting Starting fluorescence mixture mixture analysis) Example number:

It is clearly apparent from the table that the ratio of tantalum to zirconium in the deposited layers is substantially constant (within 20%) in spite of the significant change of the ratio in the starting material, the change being by the factor 5.

This characteristic of the inventive procedure constitutes an important development and progress from a manufacturing technical point of view as compared to known vapor deposition procedures. In the prior art procedures, extreme care had to be taken that the composition of the starting material was precisely adhered to in order to obtain the desired result. This is no longer necessary as long as the ratios previously referred to are being adhered to. Further, it has been found that also the other determinative factors of vacuum vapor deposition procedures can be changed within relatively wide limits. Thus, in the prior art procedures, the vaporizing speed or rate was considered a critical factor in the vaporizing of mixtures. With the same starting material, widely different condensates or deposits were obtained with different vaporization speeds. By contrast, in accordance with the invention, a reliable procedure is now available which results without great expenditure in the production of highly refractive light permeable layers of excellent reproducibility.

The following Table 2 lists a number of additional embodiments for the inventive procedure. For all these examples, the following is applicable:

The deposition was carried out in a customary vacuum vapor deposition plant. The starting mixtures were in each case heatedin a crucible to a sufficient temperature, for example, 1700 to 1800 C., so as to result in vaporization of the starting mixture. The manner of heating the mixture in the crucible is not critical and can be efiected by known ways, for example, a tungsten crucible may be used which is heated by electric resistance heating, or the heating is effected by means of an electron beam. In the latter case, the electron beam is directed onto the surface of the mixture to be vaporized which thus is directly heated to the evaporation temperature. The heat efiect of the electron beam is, as is known, the product of the current intensity of the beam and the applied potential. In all the following examples, layers were obtained which are completely homogeneous in the above indicated sense, although the deposition was carried out with a growth rate of the layers on the support of about 350 A per minute up to layer thicknesses of about 300 nm. In order to obtain hard and stable layers which are as absorption free as possible, it is recommended to heat the supports on which the layer is to be deposited prior to the deposition to a temperature of about 300 C. As pre- The layers obtained in accordance with the invention may be used for numerous purposes. For example, they are extremely suitable, as already mentioned, for the build-up of multi-layer coatings. Such multi-layer coatings, as compared to single-layer reflection-reducing coatings have the advantage that they result in a significantly stronger reduction of the reflection of a coated surface.

TABLE 2 Weight Mixture ratio Evaporation by- Exemple number:

4 Taz05:ZrOr 30:70 Resistance heated tungsten crucible. 5..- Tantalum-suboxidezZrOa 1:1 Electronbeam, 2.5 kw.

6 'Ia2O ;Zr 60:40 Resistance heated tungsten crucible. 7 Tazoszzirconiumsuboxide 1:1 Do.

8 Ta:Zr02 30:70 Electronbeam,2.5kw.

The tantalum-suboxide mentioned in Table 2, may be obtained by melting together a stoichiometric mixture of tantalum metal and tantalum-pentoxide while the mentioned zirconium-suboxide may be obtained by melting together a stoichiometric mixture of zirconium metal and zirconium-dioxide. The melting-together may be effected in advance of the vapor deposition in a preceding step to obtain the respective suboxide, or it may be effected in the vaporizing crucible proper in which event the starting mixture contains three components. In other words, and referring to Example 5, the crucible would be fed with a mixture of metallic tantalum, Ta O and ZrO while, in respect to Example 7, the mixture in the crucible would consist of Ta O Zr and ZrO As is clear from Examples 1, 2 and 3 of Table l, the inventive procedure results in the formation of layers of predetermined composition, to wit, the layer consists of oxygen, tantalum and zirconium. It is assumed that in the ideal case, the ratio of the number of atoms of tantalum to the number of atoms of zirconium is about 1:1.5. For practical requirements, however, it is not necessary to adhere to this theoretical value, but values deviating therefrom also give excellent results. Further, it has to be considered that the measurement of the ratio is subject to measuring errors. However, it has been ascertained with certainty that the atomic ratio in the inventive layer is always within a limit range and that the ratio of the number of tantalum atoms to the number of zirconium atoms is thus always between 1:1 to 1:2. Further, investigations by X-ray diagrams have indicated that the inventive layers do not contain any crystalline zirconium oxide, or if crystalline zirconium oxide is contained in them, the amount is extremely small. This explains the superior homogeneity of the inventive layers.

Further, the inventive layers may be successfully employed for interference systems with satisfactory ultraviolet permeability. This is so because the layers up to a value of about 340 nm. may be produced without exhibiting any absorption whatsoever.

The single figure shows a support having an inventive layer deposited thereon. Referring now to the drawings, the support 1, in the form of a glass plate, has deposited thereon the layer 2 consisting of tantalum, zirconium and oxygen.

What is claimed is:

1. In an optical structure having a substratum with a thin oxygen and metal containing layer deposited thereon, the improvement which comprises that the layer is light permeable and highly retracting, and essentially consists of oxygen, zirconium and tantalum, the ratio of the number of tantalum atoms to zirconium atoms in the layer being in the range of 1:136 to 1:1.7, said layer being essentially devoid of crystalline zirconium oxide.

2. The improvement as claimed in claim 1, wherein the thickness of said layer on said support is at the most about 340 nm.

References Cited UNITED STATES PATENTS 2,904,452 9/1959 Reichelt 117-406 R 3,502,502 3/1970 Elsby l17l06 R 3,259,558 7/1966 Hagiwara 117-221 X RALPH S. KENDALL, Primary Examiner M. F. ESPOSITO, Assistant Examiner U.S. Cl. X.R.

ll733.3, 169 R, 221; 252-300 

